Field of Invention
The invention relates to optical fiber cables. More particularly, the invention relates to optical fiber cables used for seismic mapping of terrestrial or underseas geological formations.
Description of Related Art
Advanced techniques for seismic mapping of underground geological formations use multiple seismic sensor boxes deployed in a large x-y array spread over the area being surveyed. The sensor boxes are typically motion sensors, for example, accelerometers. In typical methods and systems, the sensor boxes record seismic activity by converting detected motion to an optical signal. Optical signals from the seismic sensor boxes are transmitted over optical fibers to a base station, where data from the sensor box array is collected and processed. Each seismic sensor box communicates with the base station over its dedicated optical fiber.
In a typical seismic sensor box array, a main optical fiber seismic sensing cable, many meters in length, is deployed over a portion of the land or undersea area being mapped. Many cables, typically arranged in a parallel array, may be used to cover the mapped area. For undersea mapping, the array of multiple cables may be towed over a seabed by an ocean going vessel.
A relatively unique characteristic of such terrestrial seismic sensing optical fiber cables is that they are deployed and redeployed many times during the service life of the cable. This redeployment contrasts with most fiber optic cable, which typically is installed in one place and remains stationary for the service life of the cable.
In terrestrial seismic sensing applications, it is desirable to deploy the same system of cables and sensors repeatedly to map large areas, while also minimizing capital cost. It is desirable to use fiber optic cables in such systems due to their sensitivity and low power consumption requirements relative to copper-based cabling systems. Rugged conventional telecommunications-grade cables are relatively difficult to use in this type of application. Rugged conventional telecommunications-grade cables are relatively stiff and difficult to work with, and their weight and size restricts the length of cable that can be placed on a reel. Small flexible cables work better for deployment in this type of application, but small flexible cables typically require special handling by hand to protect the fibers from breaking during the hundreds of deployments required from a typical sensor cable system, thus limiting the lifetime of the system.